<style>
#inner{
	height:100px;
	width:100px;
	background:green;
	margin:auto;
}
</style>
<div style="height:200px;width:300px;background:red;">
	<div style="height:150px;width:200px;background:blue;margin:auto;">
		<div id="inner">
		</div>
	</div>
</div>